This invention relates to the production of structural components from metals such as aluminum and titanium, and their alloys, and is particularly concerned with the production of structural parts by forming using a novel means for applying pressure to a workpiece to force same into contact with the forming die.
A number of alloys exhibit superplasticity and are capable of being subjected to superplastic forming to produce parts of predetermined shapes. Superplasticity is the capability of a material to develop unusually high tensile elongation with reduced tendency toward local necking during deformation. Structures have been successfully produced from aluminum, its alloys, and from a number of titanium-based alloys by the superplastic forming (SPF) process, due to the fact that such metals or alloys exhibit superplasticity characteristics.
A requirement has existed for an improved method of forming of heavy plate in many industries. Further, there is a tendency for a conventionally formed part to spring back from the tool or die following the forming operation, resulting in the formation of a distorted part. Furthermore, parts produced by forming often have residual stresses, due to pressure or thermal gradients, and such stresses must be removed by stress relieving operations. Also, when the forming die has complex surface contours, the resulting formed part often does not have the precise matching shape of the contours of the forming die when employing conventional forming apparatus and techniques.
U.S. Pat. No. 4,087,037 to Schier, et al discloses superplastic forming and diffusion bonding apparatus, employing fluid pressure means, including a forming die having a desired contour and a cooperating flexible die matching the contour of the forming die.
U.S. Pat. No. 3,739,617 to Stejskal discloses a forming structure comprising a die with heating electrodes for receiving a metallic blank, and a heat resistant forming diaphragm for pressing the metallic blank over the hot die to form the blank into the desired part. However, such diaphragm is not a high pressure diaphragm, the forming force being created by a vacuum.
It is an object of the present invention to provide a process and apparatus for producing structural members by forming, employing a relatively simple but reliable means for applying pressure to a workpiece to expand same into contact with a forming die having a contoured surface.
Another object of the invention is the provision of a process and apparatus of the type described above, employing a pressure applying means which is operable at high temperatures for superplastic forming of a workpiece while permitting the forming to be utilized in a wide range of shapes of the forming die.
A further object is to provide apparatus and procedure of the type described above which is suitable for forming of heavy plates.
A still further object of the invention is the provision of a procedure and apparatus of the type described above for superplastically forming a metal workpiece and which avoids distortion and residual stresses in the resulting part.